| Summary: | Many flatfish species, such as Scophthalmus maximus, Paralichthys olivaceus, Solea senegalensis, are extensively cultured worldwide. In flatfishes, eye migration varies among different species and even within the same species. The genetic mechanism underlying eye migration is still unclear, and it is the key to improve metamorphosis in flatfish aquaculture and understand flatfish evolution. In this study, the high frequency of eye location variants in an artificial S. senegalensis population allowed us to investigate the eye preference during migration; completeness of the eye migration; and other related left-right asymmetric characteristics, such as asymmetrical skull remodeling and lying down on one side. The different frequencies of eye preference during migration, completeness of the eye migration, and settling side suggest that they are determined by different genotypes and imply independent evolutionary events during flatfish evolution. We found that the swimming time may be related to the completeness of eye migration, and further the migration of the eye, the more time the fish spent at the sea bottom. The left-right asymmetric degrees of not only the frontal bones and lateral ethmoids but also the jaw bone, including the premaxillary, maxillary, and dentary bones, depend on the completeness of eye migration. Finally, we proposed a hypothesis for the asymmetric origin of flatfish that provides reasonable explanations for the selective advantages of primitive flatfish species.
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